How to set a custom icon for your custom visual element in UI Toolkit?

I am creating a set of my own custom visual elements in C# to be used in ui-toolkit.
I want to show them with a custom icon when I drag them and use them in my ui-toolkit builder.
Each unity visual element has its own icon so that you get a better view in the visual tree.
So I know for sure it is possible, but I do not know how and could not find anything online.
Would appreciate if you could help me with that.